The present invention pertains to the field of reactive ion etching.
The etching of SiO.sub.2 has been performed by use of many different processes including ion milling, sputter etching, plasma etching and reactive ion etching.
Ion milling or ion etching is a process in which substrate atoms are removed by bombardment with an ion stream. Etching results from the transfer of momentum from the incident ions to the target atoms on the substrate. The ion stream in an ion mill is produced by a gas plasma, usually argon, which can be either rf or dc excited. The plasma is isolated from the milling chamber by a sequence of grids. The ions are focused and accelerated toward the target by the negatively biased grid network. Because the target is isolated from the plasma and the concomitant electron and hot atom bombardment, the target temperatures can be minimized by water cooling. Some advantages of an ion mill are; the etching is highly directional, photoresist can be used as a mask material for pattern delineation and the substrate can be tilted to etch at an angle. Some disadvantages of ion milling are: low etch rate because the sputtering process is limited by the ion beam current, sputter depths are limited due to poor selectivity between the etch rate of the mask and substrate materials, maximum etch depth is limited to approximately the mask thickness due to redeposition of material in the etched area when the etch depth exceeds the etched width, and trenching along the etch walls. Trenching and redeposition are particularly apparent when thick masks are used.
Rf sputter etching is a process in which substrate atoms are removed by bombardment with gas ions of an inert gas. Rf sputter etching is performed in a vacuum chamber containing parallel plates, across which plates rf power is capacitively coupled. The substrate is placed upon the cathode and the other plate, the anode, is grounded. The chamber is first evacuated and then backfilled to a pressure of about .about.10 millitorr. Typically, the inert gas used is argon. When the rf power is applied to the plates a gas plasma is formed and a negative bias is generated at the cathode surface because the electrons have a greater mobility than the ions in the plasma. A "no-glow" region called the dark space is formed near the cathode. This dark space typically occupies a third of the distance between the anode and cathode and has a high field. The ions in the plasma are accelerated across the dark space toward the cathode and the substrate placed thereon. The etching is directional (i.e. anisotropic) because the electric field in the dark space controls the direction of acceleration of the ions and the electric field therein is perpendicular to the cathode because of the parallel nature of the plates. Rf sputter etching usually requires the use of metal masks for pattern delineation because of the proximity of the hot plasma to the substrate. The power dissipated by the electron current provides heat, which heat prohibits the use of photoresist as a mask material. As with ion milling, sputter etch selectivity is small and this limits the etch depth. Although redeposition and trenching are difficult to prevent when large etched depths are required, the system is simpler and more easily maintained than an ion beam milling system.
Plasma etching is a process in which the substrate is exposed to a chemically reactive ionized or excited gas mixture. There is no high voltage to provide ion acceleration. The substrate is exposed to a reactive gas plasma which has been produced by either rf or dc voltages. The substrates in a typical plasma etching system are placed at the anode potential and high gas pressures of the order of 100 .mu.m are used. The plasma etching process involves low temperatures because the power required to generate the plasma is low. The low temperature permits the use of photoresist as a mask material in most situations. Because plasma etching is a chemical process rather than an ion bombardment process, etch selectivity between mask and substrate materials is high. Very deep etching can be provided with minimal mask erosion. However, unlike the ion milling and sputter etching processes described hereinabove, plasma etching tends to be an isotropic process. This isotropy causes undercutting and makes retention of pattern profiles difficult to control. One advantage, however, is that redeposition of substrate material is eliminated with plasma etching because the products of the chemical reactions are volatile and are withdrawn from the etch chamber.
Reactive sputter etching (sometimes called reactive ion etching), like plasma etching, is primarily a chemical etching process. The process is generally carried out in a standard rf sputtering station at low rf power, .ltoreq.0.3 W/cm.sup.2. Like sputter etching, with gas pressures in the range of 1 to 30 .mu.m, the substrates are placed on the cathode. The etching process is thought to be a combination of chemical reaction and ion bombardment. However, neither the relative combination of each nor the nature of the reactive species actually responsible for the etching is fully understood at present. Since reactive sputter etching is a combination of chemical and sputter etching processes, it combines some desirable features of both processes. The predominantly chemical nature of the process is responsible for high etch rates and etch selectivity. It uses a lower power than sputter etching and therefore is a lower temperature process. Undercutting, which is a large problem in plasma etching, is eliminated with reactive ion etching because of the highly directional ion acceleration across the dark space normal to the target, which effect is characteristic of low pressure ion sputtering processes. Furthermore, due to the volatility of the chemical reaction products redeposition is also eliminated.
In some cases, it is desirable to reactive sputter or rf sputter etch at oblique angles relative to the substrate normal, as is possible in conventional ion beam etching configurations. In the parallel-plate rf sputter etch geometry, if the substrates are merely angled away from the cathode, the electric field lines, and therefore, the ion trajectories, tend to be approximately normal to the surface. If the cathode surface were tilted, or if portions of it were to protrude above the line of the cathode to distort the parallel plasma geometry, the electric field lines tend to remain perpendicular to the entire cathode surface unless the substrate were sufficiently small so as not to disturb the parallel geometry.